Organic semiconductors have proven to be commercially viable alternatives to traditional inorganic electronic materials for light emitting, charge transporting, and energy harvesting applications that require large area, lightweight, and flexible active materials. High performance polymers suitable for these applications are often designed to foster extensive π-stacking in the solid state, but this characteristic can make the material prone to aggregation, crystallization, and undesired degrees of phase segregation after device fabrication. Zhokhavets, U., et al., Thin Solid Films 2006, 496, 679. In contrast, disordered or otherwise amorphous materials that exhibit thermal stability, Sivula, K., et al., J. Am. Chem. Soc. 2006, 128, 13988; processability, Brabec, C. J. Sol. Energy Mater. Sol. Cells 2004, 83, 273; Yip, H.-L.; Jen, A. K.-Y., Energy Environ. Sci. 2012, 5, 5994; and carrier mobility, Chen, T.-A.; Rieke, R. D., Synth. Met. 1993, 60, 175; Chen, T.-A., et al., J. Am. Chem. Soc. 1995, 117, 233, could be useful for such applications.
More particularly, amorphous materials are desirable for such applications as light emitting diodes and thermoelectrics. Steric interactions within or between conjugated polymers directly influence overall intrapolymer planarity and subsequent interpolymer ordering of the resulting thin-film materials. McCullough, R. D., et al., J. Am. Chem. Soc. 1993, 115, 4910; McCullough, R. D.; Lowe, R. D., J. Chem. Soc., Chem. Commun 1992, 70; McCullough, R. D., et al., Synth. Met. 1995, 69, 279; McCullough, R. D., et al., J. Org. Chem. 1993, 58, 904-912. It is not completely understood, however, how to control by design the effects of such interactions, particularly when attempting to control interpolymer stacking and crystallization. Woo, C. H., et al., J. Am. Chem. Soc. 2008, 130, 16324. Because solution processable polymers generally require some sort of solubilizing groups (often long alkyl chains pendant to the main conjugated chain), steric interactions arising from torsional strain between repeat units that contain them play an important role in disturbing the effective conjugation length of these polymers.
Several structure-property relationships have revealed the effects of solubilizing chain regiochemistry on different polymeric materials. Jayakannan, M., et al., J. Polym Sci., Part A: Polym. Chem. 2002, 40, 251; Mondal, R., et al., J. Mater. Chem. 2011, 21, 1537; Ding, J., et al., J. Polym Sci., Part A: Polym. Chem. 2011, 49, 3374; Lee, S. K., et al., J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1821. In particular, UV-vis and fluorescence spectroscopy revealed that altering only the position of pendant alkyl chains relative to various aromatic polymer backbone subunits was enough to influence planarity within the conjugated polymer backbone. The effect of poly(3-hexylthiophene) (P3HT) regioregularity on electronic properties is a particularly well studied example. Elsenbaumer, R. L., et al., in Electronic Properties of Conjugated Polymers; Kuzmany, H., Mehring, M., Roth, S., Eds.; Springer Series in Solid State Sciences; Springer: Berlin, 1987; Vol. 76, p 400; Souto-Maior, R. M., et al., Macromolecules 1990, 23, 1268; Zagorska, M., et al., Synth. Met. 1991, 45, 385; Zagorska, M.; Krishe, B. Polymer 1990, 31, 1379; Krishe, B., et al., J. Chem. Soc., Chem. Commun 1987, 19, 1476. See also, McCullough, R. D., Adv. Mater. 1998, 10, 93, for a review of poly(3-alkylthiophene) regioregularity.
When positioned in a regiorandom fashion, these alkyl chains cause the conjugated aromatic molecules to experience torsional strain and rotate out of coplanarity, thus breaking the conjugation along the polymer backbone. Hong, S. Y., et al., Macromolecules 2001, 34, 6475. Torsional strain was exacerbated when large, sterically demanding electron-accepting units, such as benzothiadiazole and diketopyrrolopyrrole, were incorporated into thiophene-based donor-acceptor polymers. Despite unfavorable torsional interactions, donor-acceptor polymers made up of components bearing relatively large molecular footprints nevertheless exhibit low optical bandgaps and increased charge transfer and/or separation than all-donor or all-acceptor homopolymers. Havinga, E. E., et al., Polym. Bull. 1992, 29, 119; Havinga, E. E., et al., Synth. Met. 1993, 55-57, 299. These studies compared the torsional interactions among planar aromatic molecules and found that, in general, when the alkyl chains were directed away from the most sterically demanding group, the polymers showed a bathochromic shift in absorption and emission spectra to lower energy due to enhanced electronic delocalization.